Engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of both gaseous and solid material, such as, for example, particulate matter. Particulate matter may include ash and unburned carbon particles called soot.
Due to increased environmental concerns, exhaust emission standards have become more stringent. The amount of particulate matter and gaseous pollutants emitted from an engine may be regulated depending on the type, size, and/or class of engine. In order to meet these emissions standards, engine manufacturers have pursued improvements in several different engine technologies, such as fuel injection, engine management, and air induction, to name a few. In addition, engine manufacturers have developed devices for treatment of engine exhaust after it leaves the engine.
Engine manufacturers have employed exhaust treatment devices called particulate traps to remove the particulate matter from the exhaust flow of an engine. A particulate trap may include a filter designed to trap particulate matter. The use of the particulate trap for extended periods of time, however, may enable particulate matter to accumulate on the filter, thereby causing damage to the filter and/or a decline in engine performance.
One method of restoring the performance of a particulate trap may include regeneration. Regeneration of a particulate trap filter system may be accomplished by thermal regeneration, which may include increasing the temperature of the filter and the trapped particulate matter above the combustion temperature of the particulate matter, thereby burning away the collected particulate matter and regenerating the filter system. This increase in temperature may be effectuated by various means. For example, some systems employ a heating element (e.g., an electric heating element) to directly heat one or more portions of the particulate trap (e.g., the filter medium or the external housing). Other systems have been configured to heat the exhaust gases upstream from the particulate trap, allowing the flow of the heated gases through the particulate trap to transfer heat to the particulate trap. For example, some systems may alter one or more engine operating parameters, such as air/fuel mixture, to produce exhaust gases with an elevated temperature. Running an engine with a “rich” air/fuel mixture can elevate exhaust gas temperature. Other systems heat the exhaust gases upstream from the particulate trap, with the use of a burner that creates a flame within the exhaust conduit leading to the particulate trap.
The rate of soot oxidation during thermal regeneration of particulate traps determines how long a regeneration event must be. One controllable factor that influences the soot oxidation rate is the temperature of exhaust gases entering the particulate trap (i.e., inlet temperature). Higher inlet temperatures may result in faster soot oxidation rates. Faster soot oxidation rates may facilitate shorter regeneration events, which may have less of an impact on fuel efficiency (e.g., a burner type regeneration device would have to burn for shorter duration and thus use less fuel). However, inlet temperatures that are too high may cause damage to the particulate trap, not only because of the high temperatures of the gases entering the particulate trap, but also because of the resulting effect on the soot oxidation rate.
Soot oxidation rate is exponentially dependent on temperature and is thus sensitive to high temperatures as well as rapid temperature increases. Soot oxidation is an exothermic reaction that produces more heat the faster the reaction takes place. Therefore, soot oxidation rates that are too high may cause production of enough heat to cause damage to the particulate trap or other parts of the system, as well as diminish the performance of one or more components of the system. Additionally, high temperatures and/or rapid increases in temperature may also result in uncontrollable soot oxidation rates (sometimes referred to as “unstable regeneration”).
Unstable regeneration may include incineration/oxidation of accumulated particulate matter that occurs too quickly, which may result in particulate trap temperatures that can be high enough to damage the filter medium and/or other components of the system. That is, when temperatures get high enough, soot oxidation rates climb, resulting in production of enough heat from the exothermic reaction to perpetuate the soot oxidation rate even more. The reaction may burn particulates out of control until the particulate matter, which is the fuel for the burn, is consumed regardless of whether any regenerative thermal input is being made (e.g., whether a burner is being fired to heat exhaust gases upstream from the particulate trap). The uncontrollable burn may result in temperatures that are high enough to cause damage and/or a loss in performance as discussed above.
Thermal regeneration may be performed periodically as opposed to constantly. For example, one such regeneration system is disclosed by U.S. Patent Application Publication No. US 2003/0145582 by Bunting et al., published on Aug. 7, 2003 (“the '582 document”). The '582 document discloses a regeneration system configured to initiate regeneration periodically by varying the transmission shift points and/or varying the ratio of engine power to battery power. Both methods have the effect of varying the temperature of the exhaust produced by the engine.
While the system of the '582 document may provide for increased particulate trap temperatures and thereby thermal regeneration, the system does not employ a control strategy that protects against unstable regeneration. The '582 document does not disclose any ramping of particulate trap temperature or incremental temperature increases. Rather, the '582 document discloses sharply increasing the particulate trap temperature to a target regeneration temperature without regard for how quickly it reaches that target temperature.
The present disclosure is directed to solving one or more of the problems discussed above.